A widely used technique for force detection and measurement employs a mechanical capacitive readout force transducer having a capacitive output proportional to the force applied. In one such mechanical transducer, one or more capacitors are formed between an instrument frame and a proof mass suspended by a flexure. A force applied to the proof mass along a particular axis will cause displacement of the proof mass relative to the frame, which varies the capacitive output of the capacitors. The force applied to the proof mass is quantified by measuring a resultant change in capacitance.
Such a micro-accelerometer is a combination of mechanical structure comprising the proof mass, suspension flexure, and fixed instrument framework, and electrical structure comprising capacitor electrodes, current-feed connections, and external circuitry forming a capacitor circuit.
More precisely, a micromachined proof mass is connected by a flexure to a fixed instrument frame forming a part of the framework of the transducer. A capacitor is formed between one or more electrode surfaces of the hinged proof mass and opposing surfaces of cooperating fixed electrodes of the instrument frame. Movement of the proof mass electrode surfaces relative to the cooperating fixed electrodes changes the value of the capacitor of the capacitor circuit. This variation in the capacitor value depends on the relative movement of the proof mass by forces, i.e., acceleration, applied to the proof mass. A change in the measurement of the capacitor therefore represents an acceleration measurement.
According to one well-known technique, the proof mass's electrodes and cooperating fixed electrodes are formed having intermeshing comb-like fingers wherein a large quantity of substantially parallel-plate capacitors are formed between the opposing surfaces of the fixed electrode and proof mass fingers.
Capacitive readout force transducers employing such intermeshing comb-like fingers have been fabricated from a body of semiconductor material, such as silicon, as microelectromechanical systems, or “MEMS,” integrated micro devices or systems combining electrical and mechanical components fabricated using integrated circuit (IC) batch processing techniques.
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics integrated in the same environment, i.e., on a silicon chip. MEMS is an enabling technology in the field of solid-state transducers, i.e., sensors and actuators. The microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Current applications include accelerometers, pressure, chemical and flow sensors, micro-optics, optical scanners, and fluid pumps. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical force sensing device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, “Method of Making an Electrostatic Silicon Accelerometer” and U.S. Pat. No. 4,945,765 “SILICON MICROMACHINED ACCELEROMETER,” the complete disclosures of which are incorporated herein by reference.
High aspect ratio MEMS or “HIMEMS” is one known process for producing such MEMS devices, including MEMS accelerometer devices. HIMEMS permits fabrication of intricate device designs in two dimensions, but requires a fixed device thickness, on the order of a cookie cutter. Capacitive readout in-plane accelerometers fabricated as HIMEMS devices may includes a proof mass suspended on flexures with a quantity of fingers formed along its edges, the fingers on the proof mass intermeshing with cooperating electrode fingers on a fixed structure to form a quantity of capacitors there between. As the proof mass moves in response to an applied force, the intermeshing fingers of the proof mass approach or recede from the fingers of the fixed structure, whereby capacitance there between increases or decreases. The change in capacitance between fingers is a measure of the force applied to the proof mass, which can be interpreted as an acceleration signal.
One problem with such capacitive readout in-plane accelerometers is that, when fabricated as HIMEMS devices, forces applied out-of-plane relative to the intermeshing fingers causes out-of-plane separation or “de-meshing” of the fingers, which decreases the inter-finger capacitance and registers as a decrease in the applied force or acceleration. This out-of-plane separation or “de-meshing” of the fingers is a device “cross-axis sensitivity” that results in a decrease of inter-finger capacitance whether the out-of-plane force or acceleration is applied positively or negatively.
FIGS. 1 and 2 illustrate one known capacitive readout in-plane accelerometer fabricated as a high aspect ratio MEMS or HIMEMS device that suffers from cross-axis sensitivity due to out-of-plane (shown as ±z axis) separation or “de-meshing” of the fingers in response to positively and negatively applied out-of-plane forces or accelerations, wherein FIG. 1 is a plan view and FIG. 2 is a cross-sectional end view taken through FIG. 1. The capacitive readout in-plane accelerometer device 1 includes a proof mass 3 suspended at two end points 5, 7 from a frame 9 formed in a substrate. The proof mass 3 is usually secured to the frame 9 by an anodic bond or adhesive indicated generally at the end points 5, 7. The proof mass 3 is suspended at each of the two end points 5, 7 by a respective pair of flexures 11, 13 for motion along the sensitive axis of the device 1, which is illustrated here as the x-axis. When the HIMEMS device 1 is a capacitive readout in-plane accelerometer, as illustrated in FIGS. 1 and 2, the proof mass 3 may be equipped with multiple comb-like fingers 15 projected outwardly from the main body of the proof mass, indicated at 3. Each of the fingers 15 cooperates with an electrode finger 17 to form one of multiple different capacitors. Surfaces of the fingers 15 in the y-z plane and cooperating surfaces electrode fingers 17, also in the y-z plane, form opposing plates of a capacitor formed there between. The cooperating electrode fingers 17 are rigidly secured to the frame 9 by an anodic bond or adhesive indicated generally at intermittent points 19, 21. The electrode fingers 17 are thereby fixed relative to the moveable proof mass fingers 15. As a positive or negative force is applied along the x-axis, the proof mass 3 and associated capacitor plates formed on the fingers 15 approach or recede from the cooperating capacitor plates formed on the relatively fixed electrode fingers 17, whereby capacitance between the cooperating fingers 15, 17 increases or decreases. In an accelerometer device, the change in capacitance is a measure of the acceleration force applied to the proof mass 3.
An in-plane acceleration or other force applied to the proof mass 3 along the sensitive or x-axis may have a cross-axis component, shown in the example as the y-axis. However, such in-plane cross or y-axis component is masked by one or both of the over-lap of the proof mass and fixed electrode fingers 15, 17; and a change in capacitance between the fingers 15, 17 on one side of the proof mass 3 being matched by an equal and opposite change between the fingers 15, 17 on the opposite side of the proof mass.
Such compensating effects are not present in HIMEMS devices for out-of-plane components of the applied force or acceleration. Because the HIMEMS process operates only on a substrate of uniform thickness, the proof mass fingers 15 and fixed electrode fingers 17 must be formed having identical cross-axis width, i.e., z-axis dimension. In practical terms, the fixed electrodes 17 cannot be formed oversized in the z-axis dimension relative to the proof mass fingers 15, whereby changes in capacitance would be eliminated for motion of the proof mass fingers 15 along the z-axis. Nor is a resultant capacitance decrease from off-set along the z-axis of the proof mass fingers 15 relative to the fixed electrode fingers 17 compensated by an equal and opposite increase in capacitance on the opposite side of the proof mass.
Rather, as illustrated in FIG. 2, a force or component of force applied out-of-plane, i.e., acting along the z-axis, causes the proof mass 3 and its associated capacitor plates formed on the fingers 15 to translate along the z-axis relative to the fixed electrodes 17 which are supported against such movement. The proof mass fingers 15 “de-mesh” or become off-set relative to the normally intermeshing fixed electrodes 17. The result of this de-meshing or relative off-set is a decrease in capacitance between the fingers 15, 17. The decrease in capacitance appears as a reduction in the acceleration or other force operating along the x-axis, thereby resulting in an inaccurate indication.
Another problem with such capacitive readout in-plane accelerometers fabricated as HIMEMS devices is that the mass 3 must be large, e.g., 1 mm-to-10 mm on an edge, to provide both sensitivity to acceleration and sufficient space for enough comb teeth to provide a useful scale factor.
Such a large proof mass 3 also deform under vibration in complex manners at undesirably low frequencies.
Furthermore, such a large proof mass 3 may also have undesirable thermal sensitivities driven by a difference in coefficient of thermal expansion (CTE) between the proof mass 3 and the frame 9 to which it is secured.